1. Field of the Invention
The present invention relates generally to improved apparatus and methods for shuffling mapping sequences in digital modulation circuits, and includes particular applications of these circuits to digital modulators of a type useful in high fidelity audio processing.
2. Related Art
Digital-to-analog converters (DACs) are used to process digital audio signals. Typically digital data signals are received from a digital replay device or over a network, such as a cable television network. The signals are then processed by a DAC in an audio amplifier, cable receiver, or other audio device to produce an analog output within a frequency range that, when connected to a transducer such as a speaker, generates human audible sounds.
DACs used in high-fidelity audio processing typical include digital modulators that convert highly over-sampled digital values from high precision (16-20 bits) to low precision (1-3 bits), with the objective of substantially eliminating noise from the human audible band.
To prepare these low precision signals for conversion to analog form, they are mapped into digital sequences to prevent parasitic elements from degrading the signal. This process is known as sequence mapping. An analog signal is then generated from the mapped digital signal and transmitted to audio reproduction equipment.
Dither signals are commonly generated in audio circuits to overcome the tendency of high-gain feedback amplification circuits to generate audible output tones (referred to as idle tones) during periods of low or zero input amplitude when the output should be low or zero. Dither signals in the form of white noise are typically introduced into the feedback circuit during periods of low input amplitude. However, this dithering function introduces a small but measurable amount of noise into the circuit and therefore reduces signal-to-noise ratios.
Known DACs are susceptible to various types of signal distortion, harmonics, dependency on past output, and generation of unwanted output at low signal input levels. These circuits achieve high fidelity output through high-precision digital signal processing. In this context the inventor has discovered that conventional sequence mapping processes may be responsible for introducing harmonic content that propagates to the output signal. Therefore, improvements in sequence mapping are desirable to improve the fidelity of these circuits.